Low friction Ta-C films with hydrogen reservoirs

Diamond-like tetrahedral amorphous carbon (ta-C) films, which contain no hydrogen and high content of sp3 bonds have a very high hardness and elastic modulus (up to 600 GPa). They have also excellent tribological properties in atmospheres that contain humidity (50% RH) or also when submerged in water. However, in dry atmospheres the co-efficient of sliding friction against bearing steel, for instance, increases to values of over 0.7. In this paper ta-C films were modified by adding hydrogen to the films. The ta-C films were deposited by using non-filtered pulsed vacuum arc deposition in the ambient pressure of hydrogen and hydrocarbon gases. A novel layered structure containing titanium layers with hydrogen and carbon have also been grown by using filtered metal arc in combination with the pulsed carbon arc. The hydrogen content and the composition of the films were measured by using time-of-flight elastic recoil detection analysis (TOF-ERDA). The hardness of the films was measured by nanoindentation. The tribological performance of the coatings were determined by using pin-on-disk tests. The counter part material was steel (AISI 52100), the normal load applied 5 N and the sliding velocity 0.02 m/s. In the modified films the hydrogen content was up to 16 at.% in ta-C and up to approximately 50 at.% in the titanium layers. A co-efficient of friction of approximately 0.2 at dry atmosphere was measured for hydrogen-containing ta-C and also for layered structures where a hydrogen-containing Ti layer was buried under a hydrogen free ta-C layer. The results indicate that the lowered co-efficient of friction is a result of hydrogen transport to the contact surfaces with the aid of surface diffusion through the pinholes of the ta-C layer